1. Field of the Invention
The present invention relates generally to analog-to-digital converters, and more specifically, to a delta-sigma analog-to-digital converter having reduced sampled reference noise.
2. Background of the Invention
Delta-sigma modulators are in widespread use in analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), in which they provide very linear behavior and simple implementation due to the reduced number of bits used in the analog signal comparison. Delta-sigma modulators can be implemented with a high level of control of the frequency distribution of “quantization noise”, which is the difference between the ideal output value of the modulator as determined by the input signal and the actual output of the modulator provided by a quantizer. The relative simplicity of the architecture and the ability to finely control the quantization noise makes delta-sigma converter implementations very desirable.
A delta-sigma modulator is constructed around an integrating circuit that integrates an input signal, and which is maintained in a stable operating range by application of a reference input, which provides a charge that cancels, on average, the charge applied by the input signal to the integrator. In switched-capacitor implementations of the ADC reference circuit, the reference charge is applied by a switched-capacitor network. A predominant source of noise in switched-capacitor delta-sigma ADCs is thermal noise sampled from the reference voltage. While a reference voltage can be made very low noise, the instantaneous thermal noise (and any other instantaneous error, such as high-frequency noise) present on the reference voltage is “captured” by the sampling switching network at the end of the reference sampling period, when the reference charge is captured on the reference sampling capacitor. Therefore, a full range of reference noise values (including peak noise values) can and will be present in the reference as applied to the ADC integrator.
The equivalent current noise due to switched capacitor sampling is governed by the following proportionality:ieq2α4kTCfSΔf,where ieq is the equivalent input current due to the switched-capacitor sampling, k is Boltzmann's constant, T is the absolute temperature, C is the capacitance of the sampling capacitor, fS is the sampling frequency, and Δf is the bandwidth of interest. Because ieq2 depends on k, T and C, the sampled reference thermal noise is sometimes referred to as kTC noise. The impact of the above-described reference thermal noise sampling is that the overall reference noise level is directly proportional to the rate of switching of the reference. As the sampling frequency is increased, the amount of signal input current increases, but the amount of applied reference thermal noise also increases. Therefore, the frequency of reference sampling should be minimized. However, the sampling frequency cannot be lowered arbitrarily, as in order to cancel the input signal, and thereby avoid saturating the integrator, a minimum reference switching rate is dictated by the input signal level, assuming a constant reference level.
Therefore, it would be desirable to provide a delta-sigma ADC that has a reduced amount of reference thermal noise and in general, reduces the error captured from the reference voltage.